Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease with worldwide incidence and no racial, ethnic, or socioeconomic boundaries. Most ALS cases are sporadic with no known inherited component. Epidemiological studies implicate some environmental and acquired factors in its pathogenesis; however, previous genetic studies have been unable to uncover the genes involved in elucidating the molecular mechanisms of sporadic ALS. We hypothesize the aberrant epigenetic regulation of DNA contributes to the neurodegeneration in ALS. Specifically, we propose that aberrant silencing of neuronal or skeletal muscle synapse maintenance genes or cell survival genes by DNA methylation could be a critical factor in the cause of neurodegeneration in ALS. Our preliminary studies of human genomic DNA obtained from ALS patients and age-matched controls reveal differential patterns of hyper- and hypomethylation. CpG island microarrays have identified the neuroligin-1 gene promoter as being hypermethylated in human ALS nervous tissue. We also find increased levels of DNA methyltransferase isoforms (Dnmt1 and 3A) and 5- methylcytosine in human ALS nervous tissue. In cell culture and mouse models, we find that enforced expression and activity of Dnmts induce neuronal degeneration. We propose to conduct the following studies. In Specific Aim 1 we will perform additional CpG microarray analyses of human ALS and age-matched control nervous tissue and skeletal muscle to determine the specific genes that are hypermethylated or hypomethylated and to validate the patterns of gene methylation using alternative methods such as methylation- sensitive-restriction-fingerprinting and also demonstrate cell-type specificities in aberrant Dnmt expression and DNA methylation. In Specific Aim 2 we will test the hypothesis that neuroligin 1 silencing participates in the mechanisms of ALS through interneuronopathy and failure of synaptic inhibition. By studying the DNA methylation and gene silencing in ALS, we could identify novel cell-based, genome-based, and physiology-based mechanisms of ALS pathogenesis that could lead to novel strategies for treating ALS.